Togaviruses represent a taxonomically distinct family of viruses which are transmitted to man and other vertebrates by a variety of bloodsucking arthropods. Among them are some of the potentially most hazardous known human pathogens. Additional togaviruses, all immunologically related within either group A or group B, continue to be discovered. We hope to gain an insight, at the cellular and molecular level, into the kinds of evolutionary pressures to which these viruses might be exposed during their repeated vector-host cycles. Accordingly, it is the objective of this project to study the replication of Sindbis (group A) and type 2 dengue (group B) viruses in cultured mosquito and vertebrate cell lines. Specifically, we shall continue to examine (1) virions isolated from mosquito and vertebrate cell cultures with regard to physical, chemical, and biological properties; (2) intracellular events in togavirus replication, e.e., RNA synthesis and polymerase, protein, lipid, and carbohydrate synthesis, and the relation of all these to viral morphogenesis; (3) the possible role of double-stranded viral RNA as an antigen in infected hosts; (4) persistent infection of cell cultures, the role of defective viral genomes and particles in its maintenance, and the mechanisms of interference observed in such cultures; (5) temperature sensitive viral mutants isolated from persistently infected mosquito cell cultures and their genetic or nongenetic interactions.